Several publications and patent documents are referenced in this application in order to more fully describe the state of the art to which this invention pertains. Full citations for these references are found within and at the end of the specification. The disclosure of each of these references is incorporated by reference.
Plants, unlike animals, do not have fur nor can they seek shelter to survive under food shortage and cold weather conditions. Consequently, they often become dormant to avoid adverse environments, such as poor nutrition, chilling temperature and drought. Dormancy is a complex state of plant development, in which the plant body exhibits little or no growth. Plants resume their growth once the conditions are favorable.
There are mainly two types of plant dormancy, e.g., forming seeds or buds. Seed dormancy has been observed for many plants species including our major crops [1-3]. Winter dormant buds are found for instance in woody plants, bulbs, rhizomes and tubers of herbaceous plants [4]. Studies on the molecular mechanisms of bud dormancy transitions in perennial woody plants have been conducted, including pear[5], oak[6], and poplar[7].
Spirodela polyrhiza, a floating aquatic monocot, develops a specific dormant organ called a turion during its life cycle, which alternates between periods of clonal propagation and dormancy. Its leaf, stem and bud are extremely compact forming a round-shaped frond, resembling a single leaf. Large numbers of Spirodela plants can be maintained in cell cultures under totally controlled medium and environmental conditions. They reproduce vegetatively through budding of fronds (growth phase) during spring and summer[8] and transition to turions (dormant phase), when there is a shortage of nutrients in the fall or when the temperature drops in the winter[9].
Noticeably, fronds perform photosynthesis and turions function as storage for starch and germinate in the following spring[10-13]. Turion cells exhibit dense intercellular space, thick cell wall and are also rich in anthocyanins[14]. Therefore, turionsprovide a unique system to study both bud and seed dormancy because they reproduce like buds without sexual hybridization but are functionally equivalent to seeds that could generate a progeny plant in the growing season. Previous studies have shown that addition of abscisic acid (ABA) into growth medium quickly leads to turion formation after 5 days of treatment in the laboratory[13, 15, 16]. Only 3 days after ABA treatment, the Spirodela primordium is irreversibly committed to turion development[15]. The ease of growth and its direct contact with water make Spirodela a model system to gain molecular insights into plant dormancy[17].
At the molecular level, some studies onturion development have already been performed. For example, the transcript level of D-myo-inositol-3-phosphate synthase is rapidly induced within 15 min of ABA application, an enzyme that plays a key role in the inositol metabolism of the cell wall[18, 19]. The expression of the key enzyme ADP-glucose pyrophosphorylase (APL) for starch production[13] is significantly changed during turion formation. Still, not much information is known about the global transcriptome profiling for turion formation in this model system.